This invention relates to a rear wheel steering apparatus for an automobile, and more particularly it relates to a rear wheel steering apparatus for an automobile having a hydraulic power steering mechanism for the front wheels.
In recent years, devices for steering not only the front wheels but also the rear wheels of automotive vehicles have been developed. In general, the direction in which the rear wheels is steered by such devices varies depending on the speed of the vehicle. At high speeds, the rear wheels are steered in the same direction as the front wheels, as a result of which the vehicle responds more quickly to changes in the direction of steering. At very low speeds, the rear wheels are steered in the opposite direction from the front wheels in order to reduce the turning radius of the vehicle, thereby making it easier to turn the vehicle around sharp corners and to park it.
Various types of rear wheel steering devices have been proposed. For example, U.S. Pat. No. 4,313,514 discloses a mechanical device for steering the rear wheels. The rotation of the steering wheel is transmitted to both the front and rear wheels by a mechanical linkage. The angle by which the rear wheels are steered is proportional to the angle by which the steering wheel is turned. An actuator controls the location of a pivot point in the linkage in accordance with the speed of the vehicle. At high speeds, the pivot point is positioned such that the front and rear wheels are steered in the same direction, and at low speeds. It is positioned such that the front and rear wheels are steered in opposite directions.
U.S. Pat. No. 4,586,581 discloses a rear wheel steering apparatus for a vehicle having a hydraulic power steering apparatus for the front wheels in which the rear wheels are steered by hydraulic actuators. The steering shaft contains two control valves disposed in series, one of which controls the flow of oil to a power cylinder for the front wheels, and the other of which controls the flow of oil to the hydraulic actuators for the rear wheels. The angle of steering of the rear wheels is controlled in accordance with the torque applied to the steering wheel.
There are a number of problems with these and other such conventional steering devices. When the rear wheels are steered in accordance with the angle of steering of the front wheels, a steering angle sensor must be provided. This is commonly in the form of a sensor which measures the rotational angle of the steering shaft or the linear movement of the rack of a rack and pinion steering gear. However, with this arrangement, there are problems with the steering shaft from the standpoint of space and problems with the rack from the standpoint of its surroundings.
On the other hand, if the rear wheels are steered in accordance with the torque applied to the steering wheel, a device for measuring the steering torque must be provided. A torque bar is generally used for this purpose. However, if the vehicle is equipped with hydraulic power steering for the front wheels, a torsion bar is already housed within a control valve in the steering shaft. If another torque bar in the form of a torsion bar is employed for use in steering the rear wheels, there end up being two torsion bars inserted in the steering shaft in series, resulting in an undersirable decrease in the stiffness of the steering shaft.
It is conceivable to employ the torsion bar of the control valve of the hydraulic power steering device for both the front wheels and the rear wheels. In this case, a torque sensor which outputs electrical signals to the control valve could be used. However, this arrangement has problems with respect to space and resistance to oil pressure of the torque sensor.
There is the further problem that conventional rear wheel steering devices do not improve the directional stability of a vehicle. If a vehicle is travelling in a straight line, as shown in FIG. 7a, and is subjected to a sudden lateral force due to a gust of wind or the like, cornering forces F1 and F2 act on the front and rear wheels, respectively, and the vehicle veers from a straight course by an angle B. In a vehicle with a conventional rear wheel steering apparatus, if the steering wheel of the vehicle is maintained in a neutral position, both the front and rear wheels remain pointing straight ahead. Therefore, the restoring moment M which acts in the direction tending to return the vehicle to its original course is small and the vehicle continues to deviate from its original course, just as in a vehicle having unsteerable rear wheels.